Chemical pressure reactors have been used at elevated temperatures for oxidation, such as regenerating carbon for a wastewater reclamation system, or for materials conditioning, such as thermal sludge conditioning, for wastewater sludge treatment, all of which require periodic ash removal. It is a troublesome operation to actuate the cleanout valve at the bottom of the reactor, particularly because a reduction in pressure to a level below its vapor pressure results in flashing, and it is the object of the present invention to provide a continuous or semi-continuous cleanout by the use of a special blowdown pot having a control valve and vents for venting vapor and water containing the ash, maintaining a liquid level in the blowdown pot. The pressure within the chemical reactor is relatively high as compared with the pressure in the blowdown pot, whereby the residue, including water and ash from the reactor, is blown downwardly into the blowdown pot.